This invention relates to a method of and apparatus for treating a feed gas stream containing hydrogen sulphide.
So-called acid gas streams containing hydrogen sulphide and carbon dioxide are formed as waste streams in, for example, oil and gas refineries. It is necessary to treat an acid gas stream so as to remove substantially all its content of hydrogen sulphide before it is discharged to the atmosphere. This removal of hydrogen sulphide is conventionally performed by the Claus process, in which part of the hydrogen sulphide content is burned in a furnace to form sulphur dioxide and water vapour; some of the resultant sulphur dioxide reacts in the furnace with residual hydrogen sulphide to form a gas mixture containing hydrogen sulphide, sulphur dioxide, carbon dioxide, water vapour and sulphur vapour, and also typically including hydrogen, carbon monoxide, carbon oxysulphide (carbonyl sulphide) and carbon disulphide. The sulphur vapour is extracted from the gas mixture by condensation, and the resulting gas mixture substantially free of sulphur vapour is subjected to a plurality of catalytic stages of further reaction between sulphur dioxide and hydrogen sulphide so as to form further sulphur vapour. The further sulphur vapour is extracted from the gas mixture downstream of each stage of catalytic reaction. A tail gas containing typically from 2 to 6 percent of the original sulphur content of the acid gas is thereby formed. The tail gas is sent for further treatment to remove substantially all the remaining sulphur compounds.
Increasingly, the environmental standards that the above-described treatment is required to meet are becoming more stringent. In some countries it is required that no more than three out of every thousand sulphur atoms that enter the process are discharged to the atmosphere. In other words, the percentage conversion of hydrogen sulphide to sulphur needs to be at least 99.7% to meet this standard. As a result, Claus plants have tended to become more complex with a greater number of catalytic stages of reaction between hydrogen sulphide and sulphur dioxide and a more elaborate tail gas clean up unit. This elaboration has added to the cost of the plant.
EP-A-0 565 316 relates to a process which makes possible a reduction in both the number and the size of the catalytic stages of reaction between hydrogen sulphide and sulphur dioxide, and in some examples can eliminate these stages altogether. In the process according to EP-A-0 565 316 some or all of the gas mixture leaving the sulphur condenser associated with the furnace is sent to a catalytic hydrogenator in which sulphur dioxide is reduced to hydrogen sulphide. Water vapour is then removed by condensation from the gas mixture exiting the catalytic hydrogenator. The resulting gas mixture depleted of water vapour is typically divided into two streams. One stream is recycled to the furnace. The other stream may be fed to a train of further but smaller, stages of reaction between hydrogen sulphide and sulphur dioxide, or in a few examples may be sent directly to an incinerator. In the examples in which the purge gas is sent directly to an incinerator, both the feed and the recycled gas streams are preheated to a temperature in the range of 100xc2x0 C. to 300xc2x0 C. It is stated that the recycle to feed ratio tends rapidly to increase with increasing percentage conversions above 98%, thereby increasing the need for preheating of the recycle and adding a requirement for increasing the size of the furnace. Accordingly such examples are not suitable for meeting a demand for 99.7% conversion of hydrogen sulphide to sulphur irrespective of the composition of the feed gas stream. It is also disclosed that hydrogen sulphide may be separated from the purge stream by washing it with a suitable amine.
U.S. Pat. No. 4,919,912 also discloses processes for recovering sulphur from a hydrogen sulphide containing gas stream in which an amine separation step is used. In the process according to U.S. Pat. No. 4,919,912 a sulphur dioxide bearing feed stream is reacted in a catalytic Claus reactor with a stream containing hydrogen sulphide and, after extraction of resulting sulphur vapour, residual sulphur dioxide is hydrogenated back to hydrogen sulphide. Hydrogen sulphide is then separated by absorption in an aqueous solution of a suitable amine and recycled to the catalytic Claus reactor as the hydrogen sulphide containing gas stream. In one class of processes according to U.S. Pat. No. 4,919,912 the sulphur dioxide bearing feed stream to the catalytic reactor is formed by combustion of a hydrogen sulphide stream. Some of the so-formed sulphur dioxide reacts with residual hydrogen sulphide to form some sulphur vapour. Removal of the resulting sulphur vapour is effected by precursory condensation upstream of the catalytic Claus reactor. This class is therefore in essence a conventional Claus process with only one catalytic stage of reaction between hydrogen sulphide and sulphur dioxide and with a tail gas clean up unit that recycles hydrogen sulphide to the catalytic Claus reactor.
The additional catalytic stage of reaction between hydrogen sulphide and sulphur dioxide in this process has the effect of reducing the mole fraction of hydrogen sulphide in the feed to the absorption unit and of thereby decreasing the amount of hydrogen sulphide which is recycled, thus making it far less advantageous in comparison with the process according to EP-A-0 565 316.
EP-A-0 798 032 relates to a modification of the process according to EP-A-0 565 316 in which catalytic stages of reaction between hydrogen sulphide and sulphur dioxide are avoided and in which downstream of the water removal stage the catalytically hydrogenated gas is compressed and subjected to fractional distillation to form a first fraction rich in hydrogen sulphide and a second fraction depleted of hydrogen sulphide. A stream of the second fraction is purged from the process, typically being sent to an incinerator, and a stream of the first fraction is returned to the furnace. This process has, however, a number of disadvantages which affect its suitability for commercial use. Firstly, because the separation by fractional distillation of hydrogen sulphide from carbon dioxide needs to be performed at below 0xc2x0 C., it is necessary in the water removal stage to have complete extraction of water vapour from the catalytically hydrogenated gas. This adds considerably to the complexity of the water removal stage, an adsorption unit being required. Further, the requirement for compression of the water-depleted hydrogenated gas stream adds considerably to the operating and capital costs of the process. It is also necessary to provide a suitable refrigeration system to provide condensation at the top of the distillation column. Finally, above about a mole fraction of 0.8 for carbon dioxide, the separation becomes difficult due to the formation of a mixture displaying azeotropic characteristics, and an unreasonably large number of theoretical separation stages would be needed if it were desired to extract carbon dioxide in a high state of purity, ie with a low hydrogen sulphide content.
None of the above described processes provides a simple method of achieving particularly high effective percentage conversions to sulphur of the hydrogen sulphide content of the feed gas stream. It is an aim of the invention to provide a method and apparatus which is suitable for this purpose; and which is capable of obtaining a conversion of 99.7% or higher when the hydrogen sulphide content of the feed gas stream is at least 70% by volume.
According to the present invention there is provided a method of treating a feed gas stream containing hydrogen sulphide and carbon dioxide, comprising the steps of:
(i) burning in a furnace part of the hydrogen sulphide content of the feed gas stream so as to form sulphur dioxide and water vapour, supplying oxygen gas to the furnace to support combustion of the said part of the feed gas stream, and reacting in the furnace resulting sulphur dioxide with hydrogen sulphide so as to form an effluent gas stream containing sulphur vapour, water vapour, hydrogen sulphide, sulphur dioxide and carbon dioxide;
(ii) extracting the sulphur vapour from the effluent gas stream by condensation so as to form a sulphur-depleted gas stream;
(iii) reducing to hydrogen sulphide the sulphur dioxide and other reducible sulphur species in the sulphur-depleted gas stream so as to form a reduced gas stream;
(iv) removing most of the water vapour from the reduced gas stream by condensation so as to form a water vapour-depleted gas stream;
(v) contacting a first part of the water vapour-depleted gas stream with at least one liquid phase absorbent of hydrogen sulphide in at least one absorber vessel so as selectively to absorb hydrogen sulphide from the water vapour-depleted gas stream and to form a hydrogen sulphide-depleted gas stream;
(vi) discharging the hydrogen sulphide-depleted gas stream as a purge stream;
(vii) desorbing hydrogen sulphide from the said absorbent so as to form at least one hydrogen sulphide rich gas stream;
(viii) returning to the furnace as a first recycle stream or streams at least part of the said hydrogen sulphide-rich gas stream; and
(ix) returning to the furnace as a second recycle stream a second part of the water vapour-depleted gas stream.
The invention also provides apparatus for the treatment of a feed gas stream containing hydrogen sulphide and carbon dioxide, comprising:
(a) a furnace arranged to burn in the presence of oxygen part of the hydrogen sulphide content of the feed gas stream so as to form sulphur dioxide and water vapour, and to allow reaction to take place between hydrogen sulphide and sulphur dioxide to form sulphur vapour, the furnace having an outlet for an effluent gas stream; containing sulphur vapour, water vapour, hydrogen sulphide, sulphur dioxide and carbon dioxide;
(b) a sulphur condenser for extracting sulphur vapour from the effluent gas stream and thereby forming a sulphur-depleted gas stream;
(c) a reactor for reducing to hydrogen sulphide the sulphur dioxide and other reducible sulphur species in the sulphur vapour depleted gas stream, and thereby forming a reduced gas stream;
(d) at least one water condenser for extracting from the reduced gas stream most of its water vapour content and thereby forming a water vapour depleted gas stream;
(e) at least one absorber vessel for contacting a first part of the water vapour depleted gas stream with at least one liquid phase absorbent of hydrogen sulphide, thereby to absorb hydrogen sulphide from the water vapour depleted gas stream and to form a hydrogen sulphide depleted gas stream;
(f) an outlet from the absorber vessel or one of the absorption vessels for discharging the hydrogen sulphide depleted gas stream as a purge stream;
(g) at least one desorber vessel for desorbing hydrogen sulphide from the said liquid phase absorbent, and thereby forming at least one hydrogen sulphide rich gas stream;
(h) means for conducting at least part of the said hydrogen sulphide-rich gas stream as a first recycle stream to the furnace; and
(i) means for conducting a second part of the water vapour depleted gas stream as a second recycle stream to the furnace.
The method and apparatus according to the invention make it possible to obtain high effective conversions of the incoming hydrogen sulphide to sulphur. This result is achieved without a provision of any catalytic stages of reaction between hydrogen sulphide and sulphur dioxide and with only one stage of sulphur removal. When the feed gas stream contains at least 70% by volume of hydrogen sulphide, even the most stringent of current environmental standards can readily be met, as it is readily possible to obtain an effective percentage conversion of hydrogen sulphide in the feed gas stream to sulphur which exceeds 99.7% or even 99.9%, i.e. more than 99.7% of all sulphur atoms in the feed gas stream are recovered by the condensation of the sulphur vapour. Lower concentrations of hydrogen sulphide can however be employed in the feed gas provided that a stable flame can be maintained in the furnace. If necessary, such techniques as preheating the feed gas, pre-concentrating it in hydrogen sulphide, or the addition of other another combustible gas to it may be employed to facilitate maintenance of a stable flame.
Combustion of hydrogen sulphide in the furnace is supported by supplying to it either commercially pure oxygen or air highly enriched in oxygen. Preferably the air highly enriched in oxygen contains at least 80% by volume of oxygen, and more preferably at least 90% by volume of oxygen. By employing commercially pure oxygen or air highly enriched in oxygen to support combustion of the hydrogen sulphide, the rate at which non-reacting gases, particularly argon and nitrogen, enter the furnace is kept down. As a result, the capacity of downstream treatment units can be kept down and the attainment of an adequate flame temperature in the furnace is facilitated. By not sending all the water vapour depleted gas stream to the hydrogen sulphide absorption stage, a further reduction in the capacity of absorber and desorber vessels employed respectively to absorb and desorb hydrogen sulphide can be achieved.
The flow rate of the purge stream or the first part of the water vapour depleted gas stream is preferably controlled so as substantially to prevent build up of inert gases in the first and second recycle streams. If such a build up did occur and were allowed to continue the method according to the invention would typically eventually reach a point at which the recycle flows were so large that it would no longer be possible to maintain a stable flame in the furnace.
Typically, the flow rate of the second recycle stream exceeds the flow rate of the first recycle gas stream(s).
If desired, the furnace may be operated with an upstream combustion stage and a downstream thermal reaction stage. The first recycle gas stream is preferably sent to the upstream combustion stage. Particularly if the feed gas stream contains ammonia, the second recycle gas stream, or at least part of it, is preferably sent to the downstream thermal reaction stage. Such an arrangement facilitates the complete destruction of ammonia in the upstream combustion stage.
It is typically found that at least 90% of all the sulphur dioxide that is formed in the furnace is reduced to sulphur by reaction with hydrogen sulphide. In a conventional Claus process the mole ratio of hydrogen sulphide to oxygen available for reaction therewith entering the furnace is normally of the order of two to one, but the conversion of sulphur dioxide to sulphur is typically less then 70%. Preferably, in the method according to the invention the mole ratio of hydrogen sulphide to available oxygen is at least 3 to 1 so as to facilitate reaction of sulphur dioxide with hydrogen sulphide. Oxygen which reacts with other combustibles, notably hydrocarbons and ammonia, is deemed not to be available for reaction with hydrogen sulphide.
It is particularly preferred that the temperature at which the feed gas stream is fed to the furnace is in the range of 0xc2x0 C. to 90xc2x0 C., more preferably in the range of 10xc2x0 C. to 60xc2x0 C. It is therefore unnecessary to provide in the apparatus according to the invention a preheater for preheating the feed gas stream. Similarly, the first and second recycle streams are preferably introduced to the furnace at a temperature in the range of 0xc2x0 C. to 90xc2x0 C., more preferably 10xc2x0 C. to 50xc2x0 C., so as to make unnecessary the provision of any recycle gas reheater.
The liquid absorbent of hydrogen sulphide is typically an aqueous solution of an amine adapted for the selective separation of hydrogen sulphide from carbon dioxide. Such amines are well known in the art and generally contain substituents which sterically hinder the absorption of carbon dioxide. A particularly preferred absorbent is methyidiethanolamine (MDEA). Other suitable absorbents of hydrogen sulphide are disclosed in U.S. Pat. No. 4,919,912. The choice of absorbent and the number of theoretical stages of absorption make it possible to manipulate the composition of, both the first recycle stream and the purge stream, thereby offering additional flexibility in the operation of the method according to the invention and thereby facilitating its optimisation. Typically, there is a single hydrogen sulphide absorber vessel and a single hydrogen sulphide desorber vessel. Alternatively, there may be two hydrogen sulphide absorber vessels arranged in series with a hydrogen sulphide lean gaseous fraction being sent from the upstream vessel to the downstream vessel for further separation, and preferably with the absorbent in the upstream vessel being different from that used in the downstream vessel, the absorbent in the downstream vessel having a higher selectivity for hydrogen sulphide than the one in the upstream vessel. Such an arrangement has the advantage of making it possible to minimise the hydrogen sulphide concentration in the purge stream.
When two absorber vessels are employed, there are preferably two desorber vessels, each being employed to form a hydrogen sulphide rich gas stream, the hydrogen sulphide rich gas streams being returned separately or together to the furnace as the first recycle gas streams.
The first part of the water vapour depleted gas stream is preferably contacted with the liquid phase absorbent at a temperature in the range of 50xc2x0 C. to 90xc2x0 C. Such an elevated temperature limits the amount of cooling that needs to be performed in step (iv) with a method according to the invention when condensing the water vapour out of the reduced gas stream.
The contact of part or all of the water vapour-depleted gas stream with the liquid phase absorbent of hydrogen sulphide is preferably performed countercurrently in a liquid-vapour contact column.
Hydrogen sulphide is preferably desorbed from the absorbent at a pressure greater than that which it is absorbed. The absorption pressure is preferably a little above atmospheric pressure, such that the hydrogen sulphide depleted gas stream may be conveyed to an incinerator or a downstream process for recovering sulphur, if any, without the need for repressurisation, and may therefore typically be in the range of 1.1 bar to 1.3 bar absolute. The desorption pressure is chosen to be sufficient for the first recycle stream to be able to flow back to the furnace without the aid of a fan or ejector. On the other hand, it is preferred to employ a fan or ejector to aid the flow of the second recycle stream to the furnace.
The purge gas is preferably sent to an incinerator in order to convert to sulphur dioxide the final traces of hydrogen sulphide. Alternatively, it may be sent to another process for recovering sulphur from sulphur containing gases.
The sulphur dioxide content of the sulphur-depleted gas stream is preferably reduced to hydrogen sulphide by hydrogen. Sufficient hydrogen may typically be formed in situ by the thermal cracking of hydrogen sulphide during step (i) of the method according to the invention. If desired, an external source of hydrogen can be provided to ensure that there is always an adequate amount of hydrogen available for reduction of the sulphur dioxide in the reactor.